1. Field of the Invention
The present invention relates to an evaporation material for forming an optical thin film, and a method for producing an optical thin film utilizing said material, adapted for forming an optical thin film of a high refractive index or a multi-layered film including the optical thin film, on a plastic lens or a disk composed of a resin material or on an optical element composed of a glass substrate and a resin layer.
2. Related Background Art
For producing an optical thin film, there have conventionally been known methods of vacuum evaporation on a glass or plastic member, utilizing an evaporation material obtained by sintering of a mixture of aluminum oxide (Al.sub.2 O.sub.3) and zirconium oxide (ZrO.sub.2) as disclosed in the U.S. Pat. No. 3,934,961, or obtained by sintering a mixture of zirconium oxide ( ZrO.sub.2 ) and titanium oxide (TiO.sub.2 ) as disclosed in the Japanese Patent Laid-Open Application No. 50-35211, or obtained by sintering a mixture of tantalum oxide (Ta.sub.2 O.sub.5) and zirconium oxide as disclosed in the Japanese Patent Publication No. 63-5723.
However, the optical thin film obtained by evaporation at a substrate temperature within a range from room temperature (ca. 20.degree. C.) to about 120.degree. C., utilizing the Al.sub.2 O.sub.3 -ZrO.sub.2 sintered mixture or the ZrO.sub.2 -TiO.sub.2 sintered mixture mentioned above does not show a sufficiently high refractive index, but only shows a refractive index in a range of 1.7 to 1.9, comparable to that of a ZrO.sub.2 film. On the other hand, a high refractive index can be attained by forming such optical thin film on a substrate heated to 300.degree. C. or higher. However, in case the substrate is a plastic optical element composed of polymethyl methacrylate (PMMA) resin, polycarbonate (PC) resin, polystyrene (PS) resin, hard polyvinyl chloride resin or polyethylene resin, or an optical element including a layer of such resin, such heating to 300.degree. C. or higher is undesirable since heating beyond the softening point Tg (80.degree.-120.degree. C.) of such resin will result in deformation.
Also an optical thin film of a high refractive index can be obtained by vacuum evaporation with a substrate having a resin layer within a range from room temperature to 120.degree. C., employing a sintered pellet containing tantalum oxide (Ta.sub.2 O.sub.5) and zirconium oxide (ZrO.sub.2) disclosed in the Japanese Patent Publication No. 63-5723 but the refractive index of such optical thin film is about 1.9 to 2.0 and is not sufficiently high.
Also, a sufficiently high refractive index of 2.0-2.1 can be obtained in an evaporated optical thin film at a substrate temperature from room temperature to 120.degree. C. by employing a single material of tantalum oxide (Ta.sub.2 O.sub.5) or niobium oxide (Nb.sub.2 O.sub.5) or titanium oxide (TiO.sub.2) and utilizing electron irradiation in vacuum with an electron gun. In such materials, however, the temperature required for obtaining a necessary saturated vapor pressure for providing a sufficient evaporation rate is higher than the melting point of the respective material, so that the entire evaporation material fuses at the evaporating operation and is scattered. Such scattered material becomes deposited as small particles onto evaporated film, thus resulting in a high defect rate in the manufacturing process.